The present disclosure generally relates to a wireless ECG device. More specifically, the present disclosure relates to a solution to modularly expand a wireless ECG device.
Electrocardiography (ECG) measures the electrical activity of the heart. It depicts the rate and the regularity of heartbeat as well as the presence of cardiac diseases or damage, arrhythmias etc. The ECG is one of the most important non-invasive diagnostic tools available to cardiologists. An ECG is measured by placing electrodes on the chest and limbs of the patient and measuring the bioelectrical potentials produced by the heart. Electrodes attached to the patient are connected by leads to an ECG monitor or communicate wirelessly to the ECG monitor for further signal processing.
Standard methods for obtaining an ECG from a subject are 3-lead, 5-lead, 12-lead or a 15-lead ECG. 3-lead and 5-lead ECGs are commonly used for routine ECG monitoring at hospitals. The 12-lead ECG monitoring provides much more information e.g. about possible cardiac ischemia, than is obtainable from the 3- or 5-lead ECG. Therefore, the 12-lead ECG is the most common of these methods and thus often referred to as the “standard 12-lead ECG”. The 5-lead measurement provides seven signals: the limb leads I, II, III, aVR, aVL, aVF and one precordial lead e.g. V5. This measurement can be done by placing one electrode on each of the patient's four limbs at the wrists and ankles and one precordial electrode on the patient's chest. The limb electrodes are referred to as left arm (LA), right arm (RA), left leg (LL), and right leg (RL). For a standard 12-lead ECG, ten electrodes are attached to a patient's body in a manner described in FIG. 1. As shown in FIG. 1, six electrodes are attached in standard positions on the chest around the heart. The standard 12-lead ECG thus provides information from the frontal plane from limb leads I, II, III, aVR, aVL and aVF and from the horizontal plane from precordial leads V1, V2, V3, V4, V5 and V6. As is commonly known in the art, the ten electrodes are connected via lead wires and resistor networks to amplifiers to record twelve separate ECG channels or leads.
The frontal leads are obtained with various permutations of the LA, RA, and LL electrodes, with the RL electrode serving as an electrical ground. The frontal leads are comprised of the potential between two of the limb electrodes: lead I corresponds to the potential between LA and RA, lead II corresponds to the potential between LL and RA, and lead III corresponds to the potential between LL and LA. Leads aVR, aVL, and aVF, referred to as the augmented leads, are comprised of the potential between one electrode and a reference input, the reference input being the average of two electrodes. For example, lead aVF is the signal between LL and a reference input, where the reference input is the average of the potentials at electrodes RA and LA.
The horizontal leads V1-V6 are obtained with various permutations of the six electrodes attached to the patient's chest, in addition to three of the four limb electrodes. Each of the six horizontal leads is comprised of the signal between the potential at the particular electrode placed on the patient's chest and the potential at Wilson's central terminal. Wilson's central terminal refers to the average potential between the RA, LA, and LL electrodes, shown simplified in FIG. 2. The three limb electrodes are connected through equal valued resistors to a common node and the voltage at this node, the Wilson central terminal, is the average of the voltages at each electrode. Each of the leads V1-V6 is compared to Wilson terminal, for example, lead V1 is the signal between electrode V1 and Wilson's central terminal.
It is not always clear when beginning the treatment what type of monitoring will be needed in the future. The patient may go under a preliminary examination and a 3- or 5-lead ECG may be applied. Sometimes further examinations are needed e.g. for eliminating certain illnesses that can be detected by recording 12-lead ECG. Changing from a 5-lead ECG to a 12-lead ECG may be time consuming and difficult. Changing the electrode set and detaching and reattaching the electrodes can take considerable time, particularly if carried out by a non-specialist. This problem could be solved by always using the 12-lead or the 15-lead ECG electrode set and cable and only using the electrodes that are needed for that particular measurement. However, the 12-lead and the 15-lead ECG measurement cables are typically thick and long which makes the nursing staff favor the smaller and more convenient 5-lead ECG measurement electrode set. In order to address this changing need, ECG systems have been developed that include a 5-lead ECG main unit and a 12-lead ECG extension unit that can be configured to operate with the 5-lead main unit. In order to combine these systems, a common mode reference signal must be shared between the systems.
U.S. Pat. No. 7,881,778, which is commonly owned with the present application, discloses a floating patient data acquisition system with an expandable ECG measurement system. The patient side acquisition units form a modular ECG measurement system which comprises a 5-lead ECG main measurement unit that is expandable by a 12-lead ECG extension unit. When the signals acquired by the 5-lead ECG main measurement unit and the 12-lead ECG extension unit are combined, a full 12-lead ECG signal is formed. The '778 patent requires a common Wilson terminal signal or value to be shared among all of the data acquisition units. For example, a 5-lead ECG can be acquired first and in case closer examination is needed, the 5-lead ECG measurements can be expanded merely by applying five additional electrodes provided by the 12-lead ECG extension unit. Since the measurement units use the same common Wilson terminal, the signals may be combined to form a full 12-lead ECG signal. The '778 patent requires that the main measurement unit and the extension unit share the same common ground potential. The common Wilson terminal signal is then referenced to this common ground potential when transmitted from the main measurement unit to the extension unit or units.
As discussed above, in existing patient monitoring devices, the number of electrocardiogram (ECG) electrodes can be chosen by the user by simply applying different number of lead wires into a connector block on the monitor. By this means, the ECG measurement can be reconfigured from 5-lead monitoring to 12-lead monitoring based on the needs of the clinicians.
In the context of body-worn ECG devices, e.g. wireless battery powered sensors or USB powered sensors, a similar modular reconfiguration is most conveniently constructed by adding on, or removing actual measurement electronics, rather than just the lead wires. By this means the device can physically be made smaller and cable connectors can be avoided. To be able to modularly add chest-leads to a main unit containing the limb-leads, a reference signal, such as the Wilson's central terminal or the right arm electrode, is typically shared between the devices. Since body-worn sensors are in contact with the human skin and therefore exposed to sweat and other fluids, reliability concerns are raised when the sweat or other fluids are present around galvanic contacts. Therefore an alternate, more reliable, solution for passing a common mode reference signal or voltage value between the main unit and one or more extension units is needed and desired.